Multi-station physical layer communication over TP cable

ABSTRACT

Wired data telecommunications networks can make advantageous use of a communications capability between and among more than two network devices. Such capabilities may be utilized in providing redundancy of data and/or inline power capabilities from a pair of network devices to a third network device receiving the redundant capability. Impedance modulated communications are provided in a wired data telecommunications network among at least a first, second and third network device coupled together via a Y device. The Y device couples the three network devices (higher order Y devices could couple more than three devices) allowing monitoring of communications and inline power provision so that one of the network devices may act in response to monitored conditions by communicating via impedance modulated communications with one or both of the other network devices.

CROSS REFERENCE TO RELATED APPLICATIONS

This Patent Application is a Divisional of U.S. patent application Ser.No. 11/154,212 filed on Jun. 15, 2005, entitled, “Multi-station PhysicalLayer Communication Over TP Cable”, which is a Continuation-In-Part ofU.S. patent application Ser. No. 11/000,734 filed on Nov. 30, 2004 andentitled, “Power and Data Redundancy in a Single Wiring Closet” in thenames of inventors Roger A. Karam and Luca Cafiero.

This patent may also be considered to be related to commonly owned U.S.patent application Ser. No. 10/961,864 filed on Oct. 7, 2004 andentitled “Bidirectional Inline Power Port” in the names of inventorsDaniel Biederman, Kenneth Coley and Frederick R. Schindler.

This patent may also be considered to be related to commonly owned U.S.patent application Ser. No. 10/961,243 filed on Oct. 7, 2004 andentitled “Redundant Power and Data Over A Wired Data TelecommunicationsNetwork” in the names of inventors Daniel Biederman, Kenneth Coley andFrederick R. Schindler.

This patent may also be considered to be related to commonly owned U.S.patent application Ser. No. 10/961,904 filed on Oct. 7, 2004 andentitled “Inline Power—Based Common Mode Communications in a Wired DataTelecommunications Network” in the names of inventors Roger A. Karam,Frederick R. Schindler and Wael William Diab.

This patent may also be considered to be related to commonly owned U.S.patent application Ser. No. 10/961,865 filed on Oct. 7, 2004 andentitled “Automatic System for Power and Data Redundancy in a Wired DataTelecommunications Network” in the names of inventors Roger A. Karam andLuca Cafiero.

This patent may also be considered to be related to commonly owned U.S.patent application Ser. No. 10/982,383 filed on Nov. 5, 2004 andentitled “Power Management for Serial-Powered Device Connections” in thename of inventor Roger A. Karam.

This patent may also be considered to be related to commonly owned U.S.patent application Ser. No. 11/022,266 filed on Dec. 23, 2004 andentitled “Redundant Power and Data In A Wired Data TelecommunicationsNetwork” in the names of inventors Roger A. Karam and Luca Cafiero.

This patent may also be considered to be related to commonly owned U.S.patent application Ser. No. 10/981,203 filed on Nov. 3, 2004 andentitled “Powered Device Classification In A Wired DataTelecommunications Network” in the name of inventors Roger A. Karam andJohn F. Wakerly.

This patent may also be considered to be related to commonly owned U.S.patent application Ser. No. 10/981,202 filed on Nov. 3, 2004 andentitled “Current Imbalance Compensation for Magnetics in a Wired DataTelecommunications Network” in the names of inventors Roger A. Karam andJohn F. Wakerly.

This patent may also be considered to be related to commonly owned U.S.patent application Ser. No. 10/845,021 filed May 13, 2004 and entitled“Improved Power Delivery over Ethernet Cable” in the names of inventorsWael William Diab and Frederick R. Schindler.

This patent may also be considered to be related to commonly owned U.S.Pat. No. 6,541,878 entitled “Integrated RJ-45 Magnetics with PhantomPower Provision” in the name of inventor Wael William Diab.

This patent may also be considered to be related to commonly owned U.S.patent application Ser. No. 10/850,205 filed May 20, 2004 and entitled“Methods and Apparatus for Provisioning Phantom Power to Remote Devices”in the name of inventors Wael William Diab and Frederick R. Schindler.

This patent may also be considered to be related to co-pending commonlyowned U.S. patent application Ser. No. 10/033,808 filed Dec. 18, 2001and entitled “Signal Disruption Detection in Powered Networking Systems”in the name of inventor Roger A. Karam.

This patent may also be considered to be related to commonly owned U.S.patent application Ser. No. 11/139,007 filed on May 25, 2005 andentitled “Detecting and Compensating for Wiring Faults on Ports of aNetwork Device Providing Inline Power” in the name of inventor Roger A.Karam.

This patent may also be considered to be related to commonly owned U.S.patent application Ser. No. 11/074,923 filed on Mar. 7, 2005 andentitled “Wiring Closet Redundancy” in the name of inventors MichaelSmith, Jeffrey Y. Wang and Roger A. Karam.

This patent may also be considered to be related to commonly owned U.S.patent application Ser. No. 11/144,094 filed on Jun. 2, 2005 andentitled “Inline Power for Multiple Devices in a Wired DataTelecommunications Network” in the name of inventors John Albert Toebes,Ping Li and Jack C. Cham.

FIELD OF THE INVENTION

The present invention relates generally to networking equipment which ispowered by and/or powers other networking equipment over wired datatelecommunications network connections.

BACKGROUND OF THE INVENTION

Inline power (also known as Power over Ethernet and PoE) is a technologyfor providing electrical power over a wired telecommunications networkfrom power source equipment (PSE) to a powered device (PD) over a linksection. The power may be injected by an endpoint PSE at one end of thelink section or by a midspan PSE along a midspan of a link section thatis distinctly separate from and between the medium dependent interfaces(MDIs) to which the ends of the link section are electrically andphysically coupled.

PoE is defined in the IEEE (The Institute of Electrical and ElectronicsEngineers, Inc.) Standard Std 802.3af-2003 published Jun. 18, 2003 andentitled “IEEE Standard for Information technology—Telecommunicationsand information exchange between systems—Local and metropolitan areanetworks—Specific requirements: Part 3 Carrier Sense Multiple Accesswith Collision Detection (CSMA/CD) Access Method and Physical LayerSpecifications: Amendment: Data Terminal Equipment (DTE) Power via MediaDependent Interface (MDI)” (herein referred to as the “IEEE 802.3afstandard”).

The IEEE 820.3af standard is a globally applicable standard forcombining the transmission of Ethernet packets with the transmission ofDC-based power over the same set of wires in a single Ethernet cable. Itis contemplated that Inline power will power such PDs as InternetProtocol (IP) telephones, surveillance cameras, switching and hubequipment for the telecommunications network, biomedical sensorequipment used for identification purposes, other biomedical equipment,radio frequency identification (RFID) card and tag readers, securitycard readers, various types of sensors and data acquisition equipment,fire and life-safety equipment in buildings, and the like. The power isdirect current, 48 Volt power available at a range of power levels fromroughly 0.5 watt to about 15.4 watts in accordance with the standard.There are mechanisms within the IEEE 802.3af standard to allocate arequested amount of power. Other proprietary schemes also exist toprovide a finer and more sophisticated allocation of power than thatprovided by the IEEE 802.3af standard while still providing basiccompliance with the standard. As the standard evolves, additional powermay also become available. Conventional 8-conductor type RG-45connectors (male or female, as appropriate) are typically used on bothends of all Ethernet connections. They are wired as defined in the IEEE802.3af standard. Two conductor wiring such as shielded or unshieldedtwisted pair wiring (or coaxial cable or other conventional networkcabling) may be used so each transmitter and receiver has a pair ofconductors associated with it.

FIGS. 1A, 1B and 1C are electrical schematic diagrams of three differentvariants of PoE as contemplated by the IEEE 802.3af standard. In FIG. 1Aa data telecommunications network 10 a comprises a switch or hub 12 awith integral power sourcing equipment (PSE) 14 a. Power from the PSE 14a is injected on the two data carrying Ethernet twisted pairs 16 aa and16 ab via center-tapped transformers 18 aa and 18 ab. Non-data carryingEthernet twisted pairs 16 ac and 16 ad are unused in this variant. Thepower from data carrying Ethernet twisted pairs 16 aa and 16 ab isconducted from center-tapped transformers 20 aa and 20 ab to powereddevice (PD) 22 a for use thereby as shown. In FIG. 1B a datatelecommunications network 10 b comprises a switch or hub 12 b withintegral power sourcing equipment (PSE) 14 b. Power from the PSE 14 b isinjected on the two non-data carrying Ethernet twisted pairs 16 bc and16 bd. Data carrying Ethernet twisted pairs 16 ba and 16 bb are unusedin this variant for power transfer. The power from non-data carryingEthernet twisted pairs 16 bc and 16 bd is conducted to powered device(PD) 22 b for use thereby as shown. In FIG. 1C a data telecommunicationsnetwork 10 c comprises a switch or hub 12 c without integral powersourcing equipment (PSE). Midspan power insertion equipment 24 simplypasses the data signals on the two data carrying Ethernet twisted pairs16 ca-1 and 16 cb-1 to corresponding data carrying Ethernet twistedpairs 16 ca-2 and 16 cb-2. Power from the PSE 14 c located in theMidspan power insertion equipment 24 is injected on the two non-datacarrying Ethernet twisted pairs 16 cc-2 and 16 cd-2 as shown. The powerfrom non-data carrying Ethernet twisted pairs 16 cc-2 and 16 cd-2 isconducted to powered device (PD) 22 c for use thereby as shown. Notethat powered end stations 26 a, 26 b and 26 c are all the same so thatthey can achieve compatibility with each of the previously describedvariants.

Turning now to FIGS. 1D and 1E, electrical schematic diagrams illustratevariants of the IEEE 802.3af standard in which 1000 Base T communicationis enabled over a four pair Ethernet cable. Inline power may be suppliedover two pair or four pair. In FIG. 1D the PD accepts power from a pairof diode bridge circuits such as full wave diode bridge rectifier typecircuits well known to those of ordinary skill in the art. Power maycome from either one or both of the diode bridge circuits, dependingupon whether inline power is delivered over Pair 1-2, Pair 3-4 or Pair1-2+Pair 3-4. In the circuit shown in FIG. 1E a PD associated with Pair1-2 is powered by inline power over Pair 1-2 and a PD associated withPair 3-4 is similarly powered. The approach used will depend upon the PDto be powered. In accordance with both of these versions, bidirectionalfull duplex communication may be carried out over each data pair, ifdesired.

Inline power is also available through techniques that are non-IEEE802.3 standard compliant as is well known to those of ordinary skill inthe art.

In order to provide regular inline power to a PD from a PSE it is ageneral requirement that two processes first be accomplished. First, a“discovery” process must be accomplished to verify that the candidate PDis, in fact, adapted to receive inline power. Second, a “classification”process must be accomplished to determine an amount of inline power toallocate to the PD, the PSE having a finite amount of inline powerresources available for allocation to coupled PDs.

The discovery process looks for an “identity network” at the PD. Theidentity network is one or more electrical components which respond incertain predetermined ways when probed by a signal from the PSE. One ofthe simplest identity networks is a resistor coupled across the twopairs of common mode power/data conductors. The IEEE 802.3af standardcalls for a 25,000 ohm resistor to be presented for discovery by the PD.The resistor may be present at all times or it may be switched into thecircuit during the discovery process in response to discovery signalsfrom the PSE.

The PSE applies some inline power (not “regular” inline power, i.e.,reduced voltage and limited current) as the discovery signal to measureresistance across the two pairs of conductors to determine if the 25,000ohm identity network is present. This is typically implemented as afirst voltage for a first period of time and a second voltage for asecond period of time, both voltages exceeding a maximum idle voltage(0-5 VDC in accordance with the IEEE 802.3af standard) which may bepresent on the pair of conductors during an “idle” time while regularinline power is not provided. The discovery signals do not enter aclassification voltage range (typically about 15-20V in accordance withthe IEEE 802.3af standard) but have a voltage between that range and theidle voltage range. The return currents responsive to application of thediscovery signals are measured and a resistance across the two pairs ofconductors is calculated. If that resistance is the identity networkresistance, then the classification process may commence, otherwise thesystem returns to an idle condition.

In accordance with the IEEE 802.3af standard, the classification processinvolves applying a voltage in a classification range to the PD. The PDmay use a current source to send a predetermined classification currentsignal back to the PSE. This classification current signal correspondsto the “class” of the PD. In the IEEE 802.3af standard as presentlyconstituted, the classes are as set forth in Table I:

TABLE 1 PSE Classification Corresponding Inline Class Current Range (mA)Power Level (W) 0 0-5 15.4 1  8-13 4.0 2 16-21 7.0 3 25-31 15.4 4 35-45Reserved

The discovery process is therefore used in order to avoid providinginline power (at full voltage of −48 VDC) to so-called “legacy” deviceswhich are not particularly adapted to receive or utilize inline power.

The classification process is therefore used in order to manage inlinepower resources so that available power resources can be efficientlyallocated and utilized.

In wired data telecommunications networks it would be advantageous toprovide additional communications capabilities between and among morethan two network devices. Disclosed below are a number of embodimentswhich enable three (or more) network devices to be connected togetherfor various purposes such as redundancy, advanced automation, and thelike. In such implementations it would be desirable to providecommunications mechanisms in order to accomplish communications amongsuch devices which do not unreasonably interfere with existingcommunications protocols used on such wired data telecommunicationsnetworks.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments of thepresent invention and, together with the detailed description, serve toexplain the principles and implementations of the invention.

In the drawings:

FIGS. 1A, 1B, 1C, 1D and 1E are electrical schematic diagrams ofportions of data telecommunications networks in accordance with theprior art.

FIG. 2 is an electrical schematic diagram of a typical Ethernet 10/100Base T connection in accordance with the prior art.

FIGS. 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 and 13 are electrical schematicdiagrams of a portion of a wired data telecommunications network segmentincorporating redundancy in power and/or data through use of a Y circuitin accordance with various embodiments of the present invention.

FIGS. 14, 15 and 16 are electrical schematic diagrams of circuits fordynamic impedance control in accordance with various embodiments of thepresent invention.

FIG. 17 is an electrical schematic diagram of an identity network inaccordance with various embodiments of the present invention.

FIG. 18 is an electrical schematic diagram of a single-pair identitynetwork in accordance with various embodiments of the present invention.

FIG. 19 is an electrical schematic diagram of a network segmentimplementing an impedance communication scheme in accordance with anembodiment of the present invention.

FIG. 20 is a graph showing the voltage of the VTERM signal(corresponding to an impedance modulation signal) plotted against time.

FIG. 21 is a graph of the difference of the voltages received at thefirst switch plotted against time for periods when VTERM is asserted andnot asserted.

FIG. 22 is a process flow diagram illustrating a first portion of aprocess for resolving communication faults in accordance with anembodiment of the present invention.

FIG. 23 is a process flow diagram illustrating a second portion of aprocess for resolving communication faults in accordance with anembodiment of the present invention.

FIG. 24 is an electrical schematic diagram of a circuit segmentproviding backup inline power to a network segment.

DETAILED DESCRIPTION

Embodiments of the present invention described in the following detaileddescription are directed at multi-station physical layer communicationover twisted-pair cable in a wired data telecommunications network.Those of ordinary skill in the art will realize that the detaileddescription is illustrative only and is not intended to restrict thescope of the claimed inventions in any way. Other embodiments of thepresent invention, beyond those embodiments described in the detaileddescription, will readily suggest themselves to those of ordinary skillin the art having the benefit of this disclosure. Reference will now bemade in detail to implementations of the present invention asillustrated in the accompanying drawings. Where appropriate, the samereference indicators will be used throughout the drawings and thefollowing detailed description to refer to the same or similar parts.

In the interest of clarity, not all of the routine features of theimplementations described herein are shown and described. It will, ofcourse, be appreciated that in the development of any such actualimplementation, numerous implementation-specific decisions must be madein order to achieve the developer's specific goals, such as compliancewith application- and business-related constraints, and that thesespecific goals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

Turning now to FIG. 2 a typical 2-pair Ethernet (10 Base T, 100 Base Tand 1000 BT if 4-pairs were used) connection is illustrated. Box 100encompasses the Ethernet port as it might exist in a network device suchas a switch, hub, router or like device. Within port 100 is a PHY orphysical layer device 102 which includes transmit circuitry 104 andreceive circuitry 106. The transmit circuitry 104 interfaces to aconnector such as an RJ-45 connector (not shown here) and through theconnector to a cable 108 which includes at least two pairs ofconductors, the Pair 1-2 (110) and the Pair 3-6 (112). The interfacebetween the transmit circuitry 104 and the cable 108 includes acenter-tapped magnetic device such as transformer T1. T1 has a PHY-sideincluding pins 1 and 2 and center tap 6, and a wire side including pins3 and 5 and center tap 4. The PHY side is also referred to as theprimary side; the wire side is also referred to as the secondary side ofthe magnetic device T1. Termination circuitry 114 provides a Vdd bias(here illustrated as +3.3 VDC) to the primary of T1. The secondary of T1is coupled to cable pair 112 which is, in turn, coupled in operation toa network device 118 which may be another hub, switch or router or a PDsuch as a Voice Over Internet Protocol (VOIP) telephone or other networkdevice.

The interface between the receive circuitry 106 and the cable 108includes a center-tapped magnetic device such as transformer T2. T2 hasa PHY-side including pins 1 and 2 and center tap 6, and a wire sideincluding pins 3 and 5 and center tap 4. The PHY side is also referredto as the primary side; the wire side is also referred to as thesecondary side of the magnetic device T2. Termination circuitry 116provides a ground bias to the primary of T2. The secondary of T2 iscoupled to cable pair 110 which is, in turn, coupled in operation to anetwork device 118. If the pairs of conductors shown belonged to a 1000Base T wired data telecommunications network segment then each pairwould transmit and receive at the same time and all four pairs in thecable would be used.

Center tap pins 4 of T1 and T2 are coupled to inline power circuitryincluding a 48 VDC power supply 120 for providing Inline Power overcable 108, control circuitry 122 and switch circuitry 124.

Turning now to FIG. 3, an electrical schematic diagram illustrates aportion of a network segment including a redundancy circuit for couplinga first port 126 of a first network device (first network device) suchas a hub, switch, router or like device and a first port 128 of a secondnetwork device (second network device) to a first port 130 of a thirdnetwork device which is a device supported by redundancy provided byport 126 and port 128. A “Y device” 132 (sometimes referred to herein asa “Y”) couples the conductor pairs (typically unshielded twisted pairs(UTP) but may be shielded twisted pairs (STP) or other types ofconductors known to those of ordinary skill in the art) so that at leasttwo pairs and, in some embodiments, all pairs of the four conductorpairs (Pair 3-6, Pair 1-2, Pair 4-5 and Pair 7-8) are coupled in a Yfashion as illustrated (i.e., each conductor is coupled so that it hasthree ends—one at each leg of the “Y”). The ends may be directlyconnected to devices or coupled to devices through connectors such asRJ-45 connectors. This approach does not use switches or switch-likeelements to select one of port 126 and port 128. Rather, it shorts eachconductor so that it is coupled to ports 126, 128 and 130 all at thesame time. It relies on the requirement that the cabling between the Ydevice 132 and the respective ports 126 (Cable 1) and 128 (Cable 2) bekept relatively short (on the order of less than about 0.5 meters for 10BaseT and less than a few inches for 100 BT and higher) note that thehigher the speed of the network, the shorter the cables have to bekept). This avoids the stub attenuator effect that would tend tosignificantly attenuate the signals carried on Cable 1 and/or Cable 2.Cable 3, which couples the Y device 132 to the port 130 may be of anynormal length appropriate for the wired data telecommunications network.

One or more identity networks may be provided at the Y device 132 inorder to provide it with Inline Power, possibly to light an indicatorlight such as a light emitting diode (LED) to indicate the presence ofInline Power at the Y device 132. Circuit blocks 134 and 136 providesuch an identity network. In Circuit block 134 associated with Pair 3-6and Pair 1-2 an identity network 138 is provided. It is coupled tocircuit blocks A (shown in detail at circuit block 142). It may includean inline resettable fuse 144 (provided for circuit protection) and apair of inductors 146, 148 which tap the DC current from the conductorpair without interfering with any AC (data) signal which may be present.A single pair identity network 150 may also be provided so that the pairmay be independently powered, if desired. Circuit block 136 isassociated with Pair 4-5 and Pair 7-8 and it is in all significantrespects the same as circuit block 134.

The identity networks 138, 140 may be a single resistor, a resistor anda pair of diodes, or other passive or active networks of electricalcomponents. Inline power may be provided to the Y device 132 and port130 from either or both of ports 126 and 128 over the Pair 3-6 and Pair1-2 conductors, or, alternatively, from port 126 over the Pair 3-6 andPair 1-2 conductors and from port 128 over the Pair 4-5 and Pair 7-8conductors (a better choice for redundancy) where applicable. An exampleof such an identity network is illustrated, for example, at FIG. 17.

Port 126 is part of a first network device 154 and port 128 is part of asecond network device 156. A router 158 couples network segment 160 to alarger network 162 such as a local area network (LAN), metropolitan areanetwork (MAN) or wide area network (WAN) such as the Internet or acorporate Intranet or the like. The link 164 coupling router 158 tonetwork 162 may be any suitable network link such as Ethernet, fiber, asatellite link, a terrestrial wireless link and the like. Router 158 maybe any device capable of providing data redundancy to first networkdevice 154 and second network device 156. The idea here is to coupleport 164 of router 158 to the network port 166 of first network device154 and port 168 of router 158 to the network port 170 of second networkdevice 156. The packets of data sent to first network device 154 shouldbe essentially the same as those going to second network device 156,except that the specific media access controller (MAC) addresses in thepacket headers will be different in most cases (although this is notrequired). Although a particular network configuration is illustratedusing a router to provide the two streams of identical signals to therespective redundant network devices (such as switches), otherconfigurations that accomplish the same result are intended to be withinthe scope of this invention. Each network device 154, 156 operates in anembodiment of the present invention like a network switch with a numberof ports. Note that the physical embodiment of network devices 154, 156may be such that they are separate line cards in a larger device,preferably running off of separate power supplies for redundancy. Theymay be (but are not required to be) built into the same box or rack as Ydevice 132 for ease of installation. FIG. 3 illustrates support for10/100/1000 Base T Ethernet (2 or 4-pairs) and power redundancy from 2sources over either 2-pairs or 4-pairs.

Turning now to FIG. 4, a 10/100-only redundancy connection is shownwhere the Y device 172 uses two of the four pairs of conductors forredundancy to port 130 and the other two pairs for communication betweenthe two redundant ports (126, 128). Pair 3-6 and Pair 1-2 are wired asin the embodiment illustrated in FIG. 3. Circuit block 174 utilizespower tap circuits 176 (which can be the same as circuit block 142 fromFIG. 3) to obtain local power (if needed). The cables from the Y device172 to ports 126, 128 are short as in the FIG. 3 embodiment. Pair 4-5and Pair 7-8 are wired differently here. In this case they are wired toprovide a direct link, via the Y device 172, between ports 126 and 128.This allows the communication of information between ports 126 and 128(such information including, but not limited to, for example: errordetection and recovery, management, status exchange, and control). Ifdesired, they may have circuit block 178 disposed across them to provideredundant inline power to Y device 172, although this is not required.In this way the network devices associated with ports 126 and 128 mayobtain such information regarding each other. In this way, one canassert that it is the master and the other the slave, the master servingas the operative network device until the slave (or master) detects aproblem and switches primacy to the slave.

Turning now to FIG. 5, an embodiment 180 of the invention for use in10/100 Base T networks is illustrated where port 130 is powered withfour pairs of conductors. Pair 3-6 and Pair 1-2 are coupled through Ydevice 182 to port 126 and port 128 of redundant network devices. Powerover Pair 4-5 and 7-8 is coupled into Pair 4-5 and Pair 7-8 of port 130via first windings 184 a and 186 a of center tapped transformers 184,186 as shown. Second windings 184 b and 186 b couple data back to theother of the two network device ports 126, 128. A single pair identitynetwork 188, 190 associated with Pair 7-8 and Pair 4-5 at the Y device182 permits the attached ports 126, 128 to ascertain the nature of the Ydevice 182. The block “A” circuits may be as shown at circuit block 192.As with the FIG. 4 embodiment, the network devices associated with ports126 and 128 may obtain status, control and link management informationregarding each other over the Pair 7-8 and Pair 4-5 link which providesa full 10/100 Base T connectivity. Transformers 184, 186 provideisolation so that data can pass through but inline power cannot. In thisway, one can assert that it is the master and the other the slave, themaster serving as the operative network device until the slave (ormaster) detects a problem and switches primacy to the slave. Data can beprovided from one network device at a time but power is available fromboth at all times (if present). Here, the inline power may be configuredas IEEE 802.3af midspan power and use the IEEE 802.3af discoveryprocess.

Turning now to FIG. 6, a modification of the FIG. 5 embodiment ispresented where the isolation transformers 184, 186 are replaced withalternative isolation transformers 194, 196. Isolation transformers 194,196 each have three windings (they are 1:1:1 transformer winding ratiotransformers). The first and second windings 194 a, 194 b on the primaryside of transformer 194 are coupled respectively to Pair 4-5 of port 126and Pair 4-5 of port 128. Third winding 194 c on the secondary side oftransformer 194 is coupled to Pair 4-5 of port 130. Similarly, windings196 a, 196 b and 196 c are coupled to Pair 7-8 of port 126, Pair 7-8 ofport 128, and Pair 7-8 of port 130. Such a three-way magnetic device hasa typical −3 db loss of signal and can have up to −6 db loss in certainconfigurations. It is used to pass inline power from the networkdevices' ports 126, 128 to the third network device's port 130 via theshorted center taps as shown and to enable each of ports 126, 128 tohear the conversation going on between the third network device port 130and the other port (i.e., port 126 can hear port 130-port 128conversations and port 128 can hear port 126-port 130 conversations inthis way. In other respects FIG. 6 is like FIG. 5.

Turning now to FIG. 7, this embodiment differs somewhat from theforegoing embodiments. Here a first network device 198 (sometimesreferred to as switch 1) has a pair of ports with connectors 200, 202(which may be RJ-45 type connectors). Connector 200 is coupled via cable2 (any length) to data terminal equipment (DTE) or switch 3 (204).Connector 202 is coupled via Cable 1 (short length) to a first port(port 1) of second network device 206 (sometimes herein referred to asswitch 2). Switch 1 (198) includes the Y circuitry for each pair andincludes PSE circuitry 208, 210 for the four conductors of Pair 3-6 andPair 1-2 as well as the four conductors of Pair 4-5 and Pair 7-8.Conventional PHY transformers 212, 214, 216 and 218 are provided andcouple the conductors to PHY 220.

Turning now to FIG. 8, a 10/100 Base T embodiment is illustrated whereeach port 126, 128 has the Pair 3-6 and Pair 1-2 pairs Y-coupled inaccordance with the present invention, but one of the ‘unused’ pairs fordata transmissions (here Pair 4-5 but can be Pair 7-8) can be used forstatus and control or even one way half duplex communication between thetwo ports 126, 128, then the other unused pair in each switch isconnected to port 130 by a pair through the Y device 222 allowing a onepair communication between the port 130 and each of ports 126, 128 forstatus and control information exchange. Such communication would behalf-duplex in nature and may use any suitable communication technology.Such pairs will help in fault discovery, recovery, management of thelink, status reporting, and feeding back inline power status at thethird port 130. It may also be used to negotiate which switch should bethe master, and the like. Providing inline power over a single pair,e.g., Pair 7-8, would entail having the proper support at the PD and thePSE since this would not be the system supported by the IEEE 802.3afstandard. Extra devices would need to be provided in the PD to supportthis option, and it would co-exist with the IEEE 802.3af standardinfrastructure.

Turning now to FIG. 9, a variant on the embodiment of FIG. 8 isillustrated. In the FIG. 8 embodiment, a 10/100 Base T redundancy schemeuses Pair 1 2 and Pair 3-6 for power and data redundancy, but should thepower supply on these pairs fail to a short circuit there would be nosimple way to recover. In the FIG. 9 embodiment, the Y device 224 iswired somewhat differently so that switch 2 (port 128) supplies powerfrom Pair 4-5 and Pair 7-8 by sending power to switch 1 (port 126) overPair 4-5 and then switch 1 (port 126) passes the power to device 3 (port130) while the other rail of the inline power from switch 2 (port 128)reaches device 3 (port 130) directly. Note how the common mode DCcurrent is passed on within switch 1. Meanwhile Pair 4-5 serves as asingle pair communication channel for status control and managementbetween switch 1 and switch 2, while Pair 7-8 serves as a as a singlepair communication channel for status control and management betweenswitch 1 and device 3.

Turning now to FIG. 10, a “backplane” or similar type of redundancyconfiguration is provided. Device 226 includes a port 126 (switch 2) anda port 128 (switch 1) which are coupled together in a Y configuration.Separate power supplies, data control and software/firmware may supporteach switch. A third port 130 associated with DTE or another switch iscoupled to device 226. Thus switch 1 and switch 2 are fully redundantand can respond to a failure of the other to provide redundant power anddata to port 130.

In order for the Y device configurations discussed herein to workoptimally, the impedance seen by the various devices must alwaysapproximate the 100 ohms of the characteristic impedance of the Ethernetcable (where another type of cable is used, its characteristic impedancemust be used). In FIG. 11 three ports are shown connected in a Y deviceconfiguration. They are port 126 of switch 2, port 128 of switch 1 andport 3 of device 130. Cable 1, Pair 3-6 couples port 126 with a Y deviceconnection jack 228; cable 2, Pair 3-6 couples port 128 with the Ydevice connection jack 228; cable 3, Pair 3-6 couples port 130 with theY device connection jack 228.

If all three devices are connected through Y device connection jack 228then the termination in one switch needs to scale up to high-impedanceacross the pairs, to keep the total termination seen into the Y deviceconnection at the third network device a total of 100 ohms. One optionincludes keeping switch 1 at 100 ohms termination impedance and forcingswitch 2 into a high-impedance (e.g., more than 1000 ohms) terminationimpedance so that switch 2 is effectively put into a receive-only mode.It is possible to allow cable 1 and cable 2 to be much longer in lengthphysically, however, in that case the impedance of the pairs must behigher (e.g., 200 ohms rather than 100 ohms) and the terminationimpedance in the switches must be higher (e.g., 200 ohms instead of 100ohms) in order for both switches to receive data. This will keep the 100ohms due to cable 3 properly terminated and allows successful datadelivery to both receivers (10/100 only). Also (referring to FIG. 16) a100 ohm to 50 ohm magnetic 228 b may be inserted in circuit block 228 a(which contains the Y device in this embodiment) to allow the matchingof the 100 ohms facing the third network device to the two-parallel 100ohm terminations seen at the switches and in cable 1 and cable 2 at thecost of a signal loss (i.e., −3 db of attenuation) but the impedancewould be matched and both switches can receive at shorter cable length(for cable 3 plus the additional cable 1 or cable 2 total physicallength).

Both switches now can leave their termination at 100 ohms. For switchbased transmission matching, where two transmitters are sharing the samepairs, a 1:1:1 magnetic replacing magnetic 228 b in the Y device, may beemployed to enable easy matching and longer cable 1 and cable 2 physicallength at the cost of about 3 db amplitude loss (up to 6 db loss in someconfigurations), but this allows both switches to leave their impedanceat 100 ohms. Other configurations in impedance matching may exist toallow the sharing of a single cable for data delivery where cable 1 andcable 2 need to be much longer in length.

When one of cables 1, 2 or 3 is disconnected from the Y connection jack228 there will necessarily be a change in impedance. A PHY equipped witha TDR (time domain reflectometer) will, in that case, detect the newimpedance imbalance either via its TDR capability or just by sensing thechange in voltage on it own transmitted signal (if it is equipped with aself-monitoring receiver circuit). It may then change its termination to100 ohms across the pairs until the unplugged cable is plugged in againas described in detail below.

In the FIG. 11 configuration cables 1 and 2 are short in length (e.g.,less than about 0.5 meter for 10 BaseT and few inches in length for 100Base T and higher), so as to not cause the other Ethernet ports to seean effective 100 ohm termination (avoiding cable length alone causing an‘effective termination’ to be reflected into the cables coupling theother 2 devices and keeping ISI (inter symbol interference) undercontrol as to not induce jitter or cause the transmit eye to loosemargin resulting in increased probability of errors due to stubreflections). Note that the maximum length of the short cables isspeed-dependent, i.e., the faster the underlying data rate, the shorterthe maximum acceptable length of the short cables.

Turning now to FIG. 12, a more detailed schematic diagram of the FIG. 11embodiment is shown having two pairs per port. Some of the details of anIEEE 802.3af standard inline power implementation are also shownalthough any inline power scheme may be used. This figure will be usedto illustrate how the termination at the PHYs is dynamically changed inresponse to the attached cables to reconfigure the impedance forsuccessful data transfer.

In FIG. 12 two sets of pairs from two different switches and the thirdnetwork device (such as a VOIP telephone) is shown for data and power.Note that in this case, power may be supplied from both switches (PSEAand PSEB) at once, i.e., a hot-standby configuration is provided whenboth are powered without worrying about which one supplies the PD withpower at a specific instant and what the percentage is, i.e., they sharethe load. For the purpose of sharing the load, the inline power sourcesare diode-OR-ed, i.e., a diode is placed in series with each one insidethe PSE (diodes D28 and D29).

One of the PSEs may see the total load while the other delivers nocurrent (since the supply voltage may be slightly different at eachPSE), thus it would need to know to keep its power up (for backuppurposes), otherwise it assumes the load is gone and it would shut downits 48 VDC supply (when in fact the load exists but it is drawing itscurrent from the other PSE). To avoid this problem the PHYs may be used.When the PHYs detect a 100 ohm impedance again after power-up and whenthe link has been operational, a check may be performed on the status ofcurrent drawn and, if it is zero, the 48 v is turned OFF, also a ‘link’down (link is a logic state within the PHY that indicates that the farend device has been absent, since the local receiver has not seen anyenergy for a predetermined amount of time in accordance with the IEEE802.3 standard) condition on a local PHY's RX pair coupled with a TDRcheck indicating an open cable (high-impedance) on the local PHY's TXpair would indicate an unplug justifying power down of the inline powersupply at the corresponding PSE. Another way to accomplish such a checkwould be to sense for the presence of any pseudo valid signals at thereceiver indicating the presence of a far end device, even though thelink state is down (i.e., the PHY does not receive the exact number ofsymbols to validate the far end device and bring the link state up).

PSEA may determine that a valid PD is attached and that it should powerup to provide inline power to the PD even if PSEB is already up asfollows. Each PSE needs to have a back off algorithm where it does notconduct IEEE discovery for a period of time so that they do not bothattempt to do so at the same time. Such back off algorithms are wellknown to those of ordinary skill in the art. Instead, PSEA performs ahigh-impedance sense of the cable in search of already-present 48 VDCinline power signals and IEEE 802.3af standard detection andclassification signals (or the like) from a possible PSEB. Should itfind regular inline power applied, PSEA would communicate with PSEB andan agreement would be reached to have the PSEA also turn its regularinline power on to provide a redundant or additional source of power toan attached third network device.

In the embodiment of FIG. 12 a 2-pair connection and the Y device 228are shown. The Y device 228 shorts: (1) TX+(TPA) of the TX pair of PSEA(also referred to as switch 1 or first network-device) to TX+(TPB) ofthe TX pair of PSEB (also referred to as switch 2 or second networkdevice); (2) TX-(TNA) of the TX pair of PSEA to TX-(TNB) of the TX pairof PSEB (pair 1 referenced here would be Pair 3-6 and pair 2 would bePair 1-2 as referenced in the RJ45 pinout); (3) RX+(RPA) of the RX pairof PSEA to RX+(RPB) of the RX pair of PSEB; and (4) RX-(RNA) of the RXpair of PSEA to RX-(RNB) of the RX pair of PSEB.

An identity network 230 is provided across the center taps ofauto-transformers 232 and 234 providing PSEA and PSEB with common modemeans to identify the connected Y device 228. The presence of theidentity network 230 in the Y device 228 allows PSEA and PSEB to knowthat they are not connected to a one-to-one single-cable connection.Should the third cable coupling the Y device 228 to the third networkdevice (i.e., port 130) be absent, then each PSE (A and B) looking intothe Y device 228 with its respective cable will see a 100 ohm impedance.Note that single-pair identity networks may be used as well but havebeen omitted from this figure for clarity.

In addition to shorting the pairs as described above, there are circuitblocks labeled RX Term and TX Term associated with each of ports 126 and128. These circuit blocks represent actively controlled terminationsthat can scale either up or down based on how many cables are attachedto the Y device 228. They are described in more detail below.

Similarly there are circuit blocks labeled FORCE_CTL associated witheach of ports 126 and 128. They are described in more detail below.

In FIG. 12 Switches S2 and S4 have been provided (they may be switches,relays, PMOS (P-channel Metal Oxide Semiconductor) or NMOS (N-channelMOS) power FETs (Field Effect Transistors), or the like) on the positivelegs of the 48 v inline power rail for the purpose of having a redundantway to shut the power down on a supply should the negative leg switchesS1 or S3 fail to function or should the local PSE controller fail tofunction. Redundant PSE controls and status circuitry or the PHY couldsense the failure in a conventional manner and then, optionally throughan optoisolator (not shown), send a signal to force S2 or S4 off whileallowing the device at port 130 to remain powered. Thus, if all ofswitches S1, S2, S3 and S4 are closed, redundant power is supplied fromswitches 1 and 2 to port 130. If S1 and/or S2 is opened then the PSEAassociated with switch 1 is cut off. If S3 and/or S4 is opened then thePSEB associated with switch 2 is cut off. Note that the 48 VDC powersupplies are not shown in FIG. 12 to improve clarity, but see, e.g.,FIG. 2). Note that control and status circuitry for switches S2 and S4may use a separate (on board) power supply and/or interface via anoptoisolator to the grounded PHY domain to receive the proper signals toshut down S2 and S4 when the need arises.

The PSE's inline power circuitry may be implemented as circuit blocks“BOX A” (associated with port 126) and “BOX B” (associated with port128) which are, in effect, in series with corresponding OR-ing diodesD29 and D28. The power from circuit blocks BOX A and BOX B may beapplied directly at the center taps of the data transformers 236, 238and 240, 242 as shown, or can be coupled through another set ofauto-transformers (windings that looks like inductors for the purposesor delivering inline power only and are ‘open’ AC-wise for data transferpurposes) as shown.

Also note the connection from the FORCE_CTL circuit blocks associatedwith ports 126 and 128 to corresponding circuit blocks BOX A and BOX Billustrate the alternative means for the location of the PSE's inlinepower control circuitry but interface to the PHY across the isolationbarrier since the 48 v supplied in accordance with the IEEE 802.3afstandard must be isolated, and we need an optoisolator (or an equivalentisolation technique) to communicate with the PHY and other circuitryaround it, i.e., the termination controls in the TX TERM and RX TERMcircuit blocks since they are referenced to system ground.

Thus the 2-pairs shown in FIG. 12 show that the Y device 228 operatessymmetrically and it effectively shorts or connects the lines designedto be 100 ohm differential impedance while handling the DC currentrequired by inline power. It also provides isolation of at least −40 dbpair to pair to insure that the Y device 228 keeps the signal from beingcontaminated by crosstalk between pairs (which would disadvantageouslyincrease the Bit Error Rate).

Should the cable to the slave be disconnected during data transmissionone RJ45 connector becomes potentially hot, i.e., the 48 VDC may bepresent from the other PSE (because the cables are shorted via the Ydevice). An LED at the connector drawing no more than approximately 2 mAmay be used to indicate the presence of such a hot connector, or othermethods may be used. Overall, if a PSE is hot-plugged into another PSEno damage should occur, and the PHY differential data and pulsetransmission will help the slave device come on-line once again.

Alternatively, diodes D30-D37 of Y device 244 in FIG. 13 could be usedto insure that there is no inappropriate hot connector or theappropriate power module could be shut down if any cable is everremoved. Or the power could be kept on but the connection monitored foran event that would draw too much instantaneous power and, in response,a quick shut-down could be implemented.

Turning now to FIG. 14, a schematic drawing illustrating the impedancechanging network using PMOS and NMOS devices to switch in a set ofresistors allowing the total impedance across a pair of conductors to bea low (e.g., 100 ohms) impedance or a high (e.g., 1000 ohms) impedance(i.e., the Y device is present and switch 2 is not loading the pairs).What is happening here is that with a 100 ohm characteristic impedancecable, the termination needs to look like 100 ohms. Conventionally thisis achieved in a single-cable system with a pair of 50 ohm terminationresistors at both ends of the link in a point to point connection. Witha Y device coupling three cables together in a Y configuration, thetotal impedance still needs to look like 100 ohms but there are twoshorted cables contributing to this on one end. With devices coupled toeach leg, one of the cables may be terminated with a high impedance.With one device missing, the cables generally need to be terminated witha pair of 50 ohm resistors.

Accordingly, circuit block 246 of FIG. 14 is an RX termination networkcircuit block in accordance with an embodiment of the present invention.This termination network is referenced to ground and includes 50 ohmresistors RA-CUT and RB-CUT along with corresponding switches MA and MB.In most cases receiver terminations are referenced to ground, but theymay be referenced to the positive rail of the power supply, e.g., 3.3VDC. Similarly, most transmitter terminations are referenced to thepositive supply rail (e.g., 3.3 VDC) but they may be referenced toground if so designed.

The termination impedance may be switched to a 100 ohm termination byturning on the MA and the MB FETs. This couples the 50 ohm resistors oneach side of the termination so that the termination impedance acrossthe port is reduced to a total of 100 ohms (i.e., no Y or third networkdevice or Y is present and this pair is in the master). This alsocorresponds to a legacy Ethernet data mode. The PHY controls the MA andthe MB transistors, i.e., the PHY detects the presence of 2 or moredevices and scales the impedance up or down, as appropriate. All of thecomponents shown may be integrated into the PHY.

Circuit block 248 of FIG. 14 is a TX termination block in accordancewith an embodiment of the present invention. It operates in essentiallythe same manner as circuit block 246, but referenced to Vdd rather thanground and uses PMOS devices that are normally on when the gate is logiclow or zero volts in stead of NMOS devices.

Circuit block 250 is the FORCE_CTL block. Note the GRX signal generatedfrom the output of NOR gate U1 in circuit block 250 can be activated bythe FORCE signal (normally logic low) driven from the inline powercircuitry through an optoisolator (not shown). FORCE is the first inputinto OR gate X1 and can be generated by another redundant circuitrunning off of some other power supply in the PHY. This provides aredundant means to force switches MA and MB OFF (taking RA-CUT andRB-CUT out of the circuit and making the termination high-impedanceagain) because the goal is to make sure the slave is high impedancesince redundancy is more critical. I.e., the configuration ofhigh-impedance mode is more important and may be the default since it isrequired for redundancy. Signal CRX is the second input into the NORGate U1 and is generated in the PHY and shown inverted (CRXBAR) goinginto the second input of the NOR gate U1. This is the normal way toswitch the port's impedance up or down when the circuitry is functioningnormally. The communication from the slave to activate the FORCE signalcan come through any available port-port communication means such ascommon mode signaling between ports, a dedicated data link between thetwo switches, wireless communication, communication over availableconductor pairs, a combination of the foregoing, and the like. A similarapproach may be used to control the FETs MD and MC (PMOS devices) inswitching the RC-Cut and RD-Cut in and out of the circuit to change thetotal impedance across the port between high-impedance and 100 ohms.Again, signal GTX can be activated either from the PHY normally via theCTX signal into second input of OR gate X1 and/or must respond to theFORCE signal going into OR gate X1 the same way the RX FETs do to insurea high-impedance termination.

The FORCE signal may be a last resort in the effort to make sure thatthe port's impedance, if impaired for any reason, is set up to allow theslave to use its high-impedance termination to keep the link going sincethe assumption is that the link was operational and there was a suddenfailure that impaired either the TX or RX circuitry on the master. Ifthe PHY in the broken switch does not respond to a command to back off(i.e., to keep its termination high-impedance and shut its transmitterdown as if it cannot receive such a command at all, or cannot executeit), then the FORCE signal would insure that its transmitter is OFF,allowing the slave's transmitter to work properly without interference.The power of the master's transmitter is FORCEd off by pulling the gateof the PMOS MTX-Vdd FET (refer to the FORCE_CTL block 250) high andthereby causing the VDD-TX power of the PHY to be turned off. This wouldmake sure that the MA and the MB FETs are also off by forcing theirgates low regardless of what the PHY is driving into the CRX pin. Thisis a redundant way to recover from a single fault in the system that hasthe potential to disrupt communications

The function of the RX Terminations (Block 246) operate in accordancewith the following CRX truth table (TABLE II):

TABLE II CRX TRUTH TABLE Vdd RA-CUT and RB-CUT are in circuit (MA, MBare ON) to provide 100 ohm termination rather than no termination -Legacy Ethernet connection or Y device is present and this pair is partof the master. GND RA-CUT and RB-CUT are both out of circuit (MA, MB areOFF) to provide high-impedance. Y is present but the current device isthe slave.

The function of the TX Terminations (Block 248) operate in accordancewith the following CTX truth table (TABLE III):

TABLE III CTX TRUTH TABLE Vdd RC-CUT and RD-CUT are both out of circuit(MC, MD are OFF) to provide no terminations - this is due to a Localpair being part of the slave. GND BA-CUT and RB-CUT are in circuit (MC,MD are ON) to provide 100 ohm termination rather than no termination ofthis is due to a Local port being legacy or a master.

The function of the FORCE_CTL block 250 operates in accordance with thefollowing truth table (TABLE IV):

TABLE IV Notes SIGNAL FORCE = GND FORCE = Vdd When FORCE = GND GTX CTXVdd GTX takes on value of CTX GRX CRXBAR GND GRX takes on value of CRXVdd-TX Vdd OFF Supply is UP

Turning now to FIG. 15, a different version 252 of the RX Terminationcircuit block 246 and a different version 254 of the TX Terminationcircuit block 248 of FIG. 14 is presented. In this version additionalredundancy is provided where the transistors controlled by the PHY areduplicated to allow the FORCE pin an independent set of transistors tocontrol should MA and MB fail, for example. Thus a single transistorfailure will not result in a loss of functionality.

While the above dynamic termination impedance is shown to have twooutcomes—100 ohms or high impedance, that is only because the casesaddressed needed a 100 ohm or high impedance solution. Those of ordinarysill in the art will now realize that the dynamic termination impedanceapproach with a higher resolution impedance step adjustment circuit maybe coupled to a bit error rate detector or other monitor and that otherimpedances may be selected to optimize measurable transmissioncharacteristics (such as optimizing the termination impedance tominimize bit error rate, and the like).

Also note that each pair, even in the 10/100 Base T case, must be ableto become a transmit and a receive, also all pairs must be able tobecome data receivers at the same time so that the “slave” may detectproblems in the “master” by listening to the cable. Additionally it isdesirable to have the ability to make all pairs data receivers at thesame time since they will have the role of listening to what the otherswitch is saying and what the third network device is communicating inreal time to detect any problems. This capability requires additionalcircuitry inside the PHY to recover the signal in real time.

Also note that the PHYs must be able to communicate with one another forthe purpose of negotiating the master-slave relationship, communicatingstatus information, communicating detected fault conditions, andarranging for a recovery from a fault condition, among other things.This may be done in a number of ways, but a single pair communicationsystem is attractive because of its ease of implementation using ahalf-duplex communication protocol between the PHYs. A wireless approachmay be used as well.

This approach may be used, for example, in certain scenarios. If thereceiver is seeing too many errors on a PHY due to a transient of somekind that broke the receiver on switch 1 or it is fully impaired forsome reason, then switch 1 may communicate with the third network deviceand switch 2 will see valid packets on the wire going from the thirdnetwork device to switch 1 but cannot on its own tell if switch 1 isseeing errors. Switch 1 may send special packets to the third networkdevice that the third network device will ignore but switch 2 willrecognize as flags passing control to switch 2 asking it to become themaster (control packets). For the purposes of shutting down thetransmitter on switch 1 and negotiating such a control transfer, asingle-pair half-duplex communication may take place. Suchcommunications could alternatively take place over wireless means, adedicated link between the two switches, common mode signaling betweenthe two switches, unused conductor pairs, and the like. Suchcommunications may, if desired, use the IEEE 802.3 Fast Link Pulsestechnology or any other communications protocol and may be half- orfull-duplex for communicating control, status and like informationbetween the two switches.

If switch 1 becomes faulty and other communications means are notavailable, impedance modulation may be used to transmit a status,initiate a status check, or request a switch to switch conversation, orrequest a slave to third network device communication. The impedancemodulation is implemented with the termination circuits discussedelsewhere herein as they can be used to modulate a termination impedanceon the transmission line which modulation can be seen by all connecteddevices. In one example, this may be implemented in one embodiment ofthe present invention with a lookup table in each switch that looks forthe termination changing from say 100 ohms to high-impedance somepredetermined number of times in a row, telling the slave, in effect, totake over. Of course this process would time out after a few seconds ifthe termination suddenly is disrupted and does not go back to itsoriginal state. In another example this may be implemented as follows.After the link is operational the master is terminated with 100 ohms,and the slave is left without a 100 ohm termination. If switch 2 is theslave and decides that it wants to communicate with switch 1, it maytake its impedance back to 100 ohms “upsetting” the termination, a backand forth change from high-impedance to 100 ohms in a predeterminedmanner (e.g., number of times) to signal the start of a half-duplex PHYto PHY communication. The third network device can detect this bylistening to its own transmitted signal and it will be designed to backoff (i.e., not transmit or expect packets while keeping its link statusup) while that communication takes place between the first two devices.The third network device may also be designed to recognize suchcommunications for the purposes of modifying its own behavior. It mayback off, stop transmission, transmit and wait for a reply, or manageits own communications keeping its link status signal valid, allowingthe switches to negotiate status, control and order any actions that maybe needed. It may listen in to get out of such a mode at the end of theswitch-to-switch communication, use a time out designed for such apurpose, or get a specific link available from either switch. Also thethird network device may change its RX impedance to request aconversation, but in the case of a long cable the “termination” effectmay render the detection such a request harder, the third network devicemay still use a special signal (e.g., special data patterns) to flag theslave that it wants a direct communication with the slave, and for theslave to initiate such a conversation it would change its impedance toflag such a request.

Turning now to FIG. 17, a configuration of an identity network 262 whichmay be used with some of the circuits of the present invention isillustrated. Identity network 262 is a special PD that is ON (i.e., itdraws a few mA to bias the series diodes (FIG. 13) ON, allowing the dataAC (alternating current) signal through) when the PSE power is “OFF”. Inthis case the PSE still supplies low DC current to permit the biasing ofthe diodes above a few volts (i.e., about 5 VDC or less). This PD ishigh-impedance allowing the easy discovery of the conventional IEEE802.3af 25k identity network (25,000 ohm resistor) in the PD or thirdnetwork device. If the third network device is not a PD then the PSEwill supply such small DC current at low voltages to keep the diodes lowimpedance for data transmission. The threshold detect circuit block 264senses an applied voltage above about 5 VDC (i.e., above idle) and, inresponse, opens switch 266 so that the circuit looks like a highimpedance when voltages above about 5 VDC (e.g., discovery,classification or regular inline power) are applied. Below that voltagelevel (i.e., at idle), the switch is closed exposing the load to thesmall amount of power available. If desired, the threshold detectcircuit 264 may close switch 266 again when normal inline power (around48 VDC) is applied. This can assist in making the identity networkdiscoverable and avoiding conflict during the discovery andclassification stages prior to application of regular inline power. Thethreshold detect circuit may be implemented with resistors and zenerdiodes and the switch may be implemented with a depletion-type FET withbreakdown protection circuitry. Other approaches will now be apparent tothose of ordinary skill in the art.

If either PSE on the wire goes high while the other tries to determinethe presence of the Y device, it would see a high impedance also (sincethe PSEs are analog OR-ed). For this purpose the PSE controller must beable to sense the voltage on the wire, i.e., to see if the 48 VDC ispresent, or if there is a discovery or classification cycle taking place(i.e., a voltage greater than about 5 VDC is present). In that case itknows to back off and try later. Such back off timing may be a value oftime that is predetermined or agreed upon among the devices, or the PSEmay monitor via its high-impedance sensing the wire voltages and seizethe wire for its own search for a PD and the Y device's common modeidentity network at the first chance where the cable voltage drops belowthe right value, of course at that instant, the Y device acting as a PDwould draw a pre-determined value of DC current from each switch thatthey can both measure. This can also be handled by any other availablecommunication system including wireless and common mode.

Another function of this PD is to flag the presence of the Y device toboth switches by using its unique identity network, i.e., the current itdraws, that would go to zero above a few volts (i.e., above idle).

In FIG. 18 an example of a single-pair identity network 268 as may beused in some of the preceding circuits is illustrated in more detail.The identity network 268 includes a pair of low-capacitance zener diodesor equivalent circuitry 270 coupled to a pair of conductors through afirst resistor 272 and a second resistor 274. Such a network 268 (aswell as others) would allow a TDR (time domain reflectometer) or similartechnique to detect the presence of something other than a 100 ohmimpedance when a special signal is applied, flagging the presence of aspecial device (i.e., the Y device) on the wire. Note that such anidentity network may be unique to each pair. In one case resistors 272and 274 may be 10 ohm resistors in series with two-zener diodes 270 and271 and the PHY sends a pulse that is much higher than the zener voltagein addition to a diode drop causing the breakdown and the attenuation tooccur across the pair. The PHY in the switch receives its owntransmitted signal and it can detect the presence of the Y device bysensing the drop in the amplitude of the voltage applied to discover theY device. If only a 100 ohm termination was present the signal would beunmodified. Such a check may be performed periodically at lowerfrequencies when the link is down, or it can be initiated once a 100 ohmtermination is detected. Note that under normal power and data operationthis network is high-impedance and low capacitance and therefore it doesnot affect the data. For uniqueness, various different zener diodevoltages, combinations of zener voltage equivalent circuitry and diodesmay be used as desired.

In operation, the Y device must be discovered by both switch 1 andswitch 2. The Y device acts as a fully symmetrical connection—in someembodiments it simply shorts three RJ45 connectors together on aconductor by conductor basis. The Y device may be discovered as follows.When configured properly (all devices are connected), the Y devicecauses an impedance perturbation at each switch's PHY on both the RX andTX pairs. The TX pairs would detect such perturbations and order thelocal impedance adjusted on all pairs (i.e., if both switches have a 100ohm termination, a valid third port with a cable attached can cause theeffective impedance to change to 50 ohms and is detected in bothswitches configured specifically to look for this condition, signalingthe presence of a Y device. If only two devices are attached, then aregular point-to-point Ethernet connection is present. If three devicesare attached with very long cables, then the Y device approach is notapplicable and other schemes may be used to avoid the stub attenuationeffect and keep a point to point communication going. If two networkdevices are connected to the Y device with relatively short cables andthere is a relatively long cable not terminated and not coupled toanything, then the signal will be loaded down due to the effectivetermination presented by the long cable (approaching 100 ohms) andtherefore, lowering the speed to 10 Base T and/or using pulses such as,for example, the Ethernet Fast Link Pulses would help in this situationto maintain communication between the two network devices. This wouldoccur if, for example, the two network devices were providing redundantpower and data to a VOIP telephone and the VOIP telephone were suddenlydisconnected and the two network devices needed to communicate with oneanother to decide how to respond to the unplug event.

Alternatively, the Y device may be discovered by having each switchperform a common mode discovery process to look for a special signaturethat resembles that of the identity network of FIG. 17 (low impedance ator below about 5 VDC (i.e., idle) and high-impedance above that level(i.e., above idle)—other voltage inflection points could be selectedinstead of 5 VDC). Note that if switch 1 and switch 2 are coupled via astraight cable the impedance would look no different than the Y devicearrangement unless a single pair identity network (e.g., FIG. 18) isused across one or more pairs in the Y device along with the common modeidentity network.

Where two redundancy-supporting switches (switch 1 and switch 2) areattached first to a Y device and the third network device is not at thatpoint in time connected (i.e., the connector is open), since eachtransmitter on every PHY is capable of receiving packets/pulses(transmitters are joined together by the Y device). Special pulses (suchas those used for Auto-negotiation, proprietary pulses, and/or a mixtureof both) may be exchanged by the switches to let the switches figure outthat they are tied together. Also, the switches are free to operate as adata network node at this instant or in the absence of a third networkdevice because, from a data perspective, the switches cannot tell thatthere is anything special about this link other than they both supportredundancy and that they are both certain types of switches and thatthey can sense the presence of the Y device. So, in the absence of athird network device, the switches are free to exchange data. If a longcable is attached at the third interface of the Y device but is notcoupled to a third network device at its far end, then the switch-switchcommunication may still be possible, but the switches might need toadjust their termination impedances to appropriate values as the longcable may cause an effective termination. The effective cabletermination is an indication that a Y device is present. It is up to thePHYs in both switches and the PSEs in both switches to see the thirdnetwork device as it gets plugged in. Both switches are capable ofcommunicating while ignoring the third network device and at times theymay back off and look for a third network device to see if it wasrecently attached. They may do this by looking for a terminationimpedance change (as with a TDR) or by looking for communication signalsfrom the third network device.

Where one redundancy supporting switch and a third network device arecoupled to the Y device and the other redundancy supporting switch isabsent for some reason and the third network device is a PD/DTE typedevice such as an Internet telephone, the port of the Y device coupledto the first switch and the port of the Y device coupled to the PD/DTEmay operate like a normal network link, unless the switch is configured(via software/firmware) to wait for information exchange from the otherswitch so it will keep the link down or allow it to come up onlyconditionally for a period of time awaiting the availability of theredundant switch. Such operation can be based on a special PD class ofthe third network device (discoverable through the discovery andclassification process) that tells the attached switch to wait and giveit redundancy or nothing, or possibly ignore redundancy (e.g., in casesome repair needs to be performed, go ahead and supply the power anddata). There is a potential problem if the second network device isunplugged while the third port is powered—it risks a hot connectionsuddenly at the open port of the Y device. Also the presence of inlinepower in the absence of the Link integrity routine or the plug-in of thesecond switch causes the second switch not to discover the common modesignature of the Y device. The use of the single-pair identity networkwould help solve this problem, along with a PSE designed to back offperiodically to check for a hot cable (i.e., the presence of 48 VDCand/or discovery voltages). Diode control as in FIG. 13 would also solvethis problem. It is best that no inline power be supplied unless theport on the Y device for the third network device is properly terminatedand the switches have negotiated the setup. One way to alleviate thisproblem is to avoid powering devices unless both switches can executetheir Link Integrity routine successfully ahead of powering the link.Another approach would be to remove inline power the instant animpedance disruption or a TDR flag happened.

If a legacy PD that does not require redundancy is attached to a port ofthe Y device, then a switch attached to any of the other ports on the Ycan be configured to operate normally or wait. While one of the ports isnot used, the attached switch is free to try and collect informationabout the attached device, i.e., whether it has a PD, its class, andpossibly test the link. Also it may opt to run the link temporarilyawaiting the presence of the redundant switch and send out messages to anetwork control point requesting assistance.

Where two redundant switches are present and a third network device ispresent and it is a switch/PSE the two redundant switches will seesignals coming from the third network device and in response they willnegotiate as to who will start a “link integrity routine”. First, a TDRin the PHY of each switch can measure the total cable length between thetwo switches to verify that they are of acceptable (short) length. Theexchange between the two switches allows for a specific random orsystem-based codeword to be generated (it could, for example, be theday's date and the port number/device ID coded in binary, hexadecimal,or the like, sent as pulses, as well as anything else) and shared amongdevices to be stored in permanent memory as a password to insure that nounauthorized use of the connection is made, since data may be duplicatedto both devices and to act as a “soft identity token” allowing a quickerrecovery and boot up of the disconnected device as it attempts to comeback on line. Then each switch would take control of the link, provideinline power to the third network device if needed, and exchange datawith the third network device (the third network device may berequested, for example, to report if it is receiving error free at thespeed and duplex chosen, and different data patterns may be sent duringthis test for testing purposes), while the other switch would be in a“monitor mode” (i.e., it would listen to the conversations from thefirst network device to the third network device and from the thirdnetwork device to the first network device in real time). In this modeall pairs on the idle switch may act as receivers in accordance withsome embodiments of the present invention, it would also check theamplitude of the inline power voltage and possibly communicate with boththe PD or third network device and the PSE using common mode signalingmethods.

After some data exchange to check the link from one of the switches(e.g., switch 1) to the third network device, switch 1 sends a specialpulse passing control to the second switch (having the role of slave atthis time), so as to have the first switch back off into monitor modeand allow the second switch to start its link integrity routine. Oncethe second switch has completed its link integrity routine, the twoswitches exchange some pulses to agree which will take on the role ofslave and which will take on the role of master. Such agreement may bebased on inline power requirements (i.e., one switch may have much moreinline power to offer or data traffic requirements), or the agreementmay be forced by software/firmware, a setup by a system administrator,or by default. The third network device may request service from eitherswitch 1 or switch 2 automatically or via user-induced selection (i.e.,pre-configured software/firmware or a physical or virtual pushbutton).When such a request is made, all devices have to agree and acknowledgebefore the change takes place. When a problem presents itself, a useroption via a software menu or a physical or virtual pushbutton at thethird device (e.g., a VOIP Phone), may allow a user to select thealternate source of power and or data. Such a request may be transmittedusing any available communications means, such as common mode over theconductors of the wired data telecommunications network, dedicated link,wireless link, impedance modulation, and the like. The communicationsmeans used may optionally be based on the type of failure or the reasonfor the failure.

Once the agreement is reached, one switch would go to monitor mode andthus perform the role of slave and listen to some or all conductor pairsat once and possibly it can have its inline power active (on) to providesimultaneous backup inline power or hot-standby backup inline power. Itdoesn't matter whether the PD (third network device) draws its currentfrom one switch or both so long as it can never drop power totally ifone power supply in a switch faulted. The PD may draw its current over2-pairs or 4-pairs.

If a fault occurs on the master, the slave would sense it via themonitor mode and it would change its transmit circuit into a transmitteragain and send a special sequence of pulses to the transmitter of themaster instructing it to back off. If the master cannot receive such amessage because it has a broken transmitter (or simply does not respond)a termination impedance signal may be sent (as described in detailabove) for the purpose of getting this message to the master. Themessage could additionally or alternatively be sent by a dedicated datalink or a common mode signaling technique. If necessary the slave couldshut off the power to part or all of the master switch to get it to stoptransmitting.

Another purpose for running a dedicated connection between the tworedundant switches (in the case of 10/100 Base T such a connection canbe over the same cable using the otherwise unused Pair 4-5 and Pair 7-8pairs and in the case of 1000 Base T or higher a dedicated connectionallows us to actively and continuously test the whole setup. In such anarrangement, the slave could send some special packets (test packets) tothe master, which, in turn, would pass them on to the third networkdevice, which in turn would pass them on to the master switch while theslave is in monitor mode so it can see its own request go across thepairs thus testing the whole setup including the third network device.The slave may opt to do this at periodic intervals or when it senses nodata transfer on the wires. Again, if the link between the two switchesfails to force the master to back off (e.g., the switch's software nolonger is up), the inline power communication or other communicationsmeans could be used to force the master to shut the PHY down but to keepthe dynamic termination impedance circuitry operating so that itpresents the proper value. In another embodiment both the master and theslave on their own would send occasional special packets (statuspackets) dispersed among normal data packets reassuring the slave thatthey are both up. This may be detected using either the detection of themissing ‘well and alive’ status packets that help the slave act like awatchdog, or using the approach where the slave instructs the master totalk to the third network device while the third network device sendsinformation back to the master. In a similar manner using common modecommunications, each PSE and a PD may be doing periodic or request-basedvoltage and or current modulation to deliver status, management and orcontrol messages via power connections about the inline power state onthe wire. For example PSE 1 may send a signal to the PD to send back thevalue of the current consumed by the PD at one instant. Such a check maybe used to calculate the dissipated power in the cable and or to see ifthe PD is using any current from PSE 2 since PSE 1 knows how the currentbeing drawn out of its power supply it can calculate the difference.Alternatively, a communication may be used to relay messages to PSE 2about the status of PSE 1 and vice versa.

Another feature to help troubleshoot and/or isolate problems should theyoccur would be to allow packet loopback in each device at the PHY level.This would be started either upon software/firmware command, or by thePHY's detection of a special test packet, data pattern, or signal. Thiswould help insure that the physical layer is in proper shape when theslave can test the physical layer alone and can determine if the problemis in the software of either the master or the third network device.Such a loopback can take place periodically when data is not present orupon command.

In one embodiment of the present invention, the Ethernet (or other wireddata communication network) termination impedance may be used forcommunicating status, reporting a problem, initiating a request such asa request for a speed change or data source change, or inline powerrelated, such as turn inline power on or off, and the like. Thisapproach may be used, for example, in a situation such as that depictedin FIG. 19. In the network segment 300 depicted in FIG. 19, a first port302 of a first switch network device such as a data communicationsswitch 304 is coupled to a corresponding port 306 of a third device 308.Note that one pair of wires is illustrated in FIG. 19 for simplicity butmultiple pairs may be present. A first port 310 of a second networkdevice such as a data communications switch 312 is coupled to the wiresconnecting the first switch with the third device via a directconnection as shown at nodes 314, 316.

This approach is not standard but will work, as pointed out above, ifthe wire lengths from the nodes 314, 316 to the second switch are keptrelatively short. Ethernet is a point-to-point communications connectionbetween and among two devices over one cable so the data is active onone pair of wires between only two devices at one time. With threedevices connected as shown in FIG. 19 the logical connection can onlyreally exist between one pair of them at any moment and thus a mechanismfor requesting a device to “back off” for a time is required to enablethe formation of a connection between the other pair of devices. Forexample, in this case, the desire would be to have the connectionbetween the first switch and the third device suspend while the secondswitch (which was monitoring communications between the first switch andthe third device) attempts to establish communications with either thethird device or the first switch.

We'll call the second switch 312 the non-active switch and the firstswitch 304 the active switch. In order for the non-active switch 312 totake control of the link, it would need to sense a condition (such as afault) that would justify taking control. Such faults might include datafaults, inline power faults, and the like. In order for it to activelylisten to the link, its termination circuitry is not grounded, i.e., itis in a high-impedance state with FET transistors 318, 320 set to OFFand its PHY transmitter circuitry is set to act as a receiver only towatch the packets on the line. If it senses a “fault”, then it needs toask the active switch 304 to back off (since in this scenario there isno desire to establish a connection between the two switches). This maybe accomplished by forcing the termination circuitry on the first port310 of the second (non-active) switch 312 from a high-impedance state toa 50-ohm impedance state by setting transistors 318, 320 to ON (e.g., byasserting the signal VTERM). Since Ethernet switches normally monitortheir own signals with a built-in transmit-side receiver in the PHY, theactive switch 304 will detect this impedance change and, withappropriate programming, will know to back off and allow the non-activeswitch 312 to establish a connection with third device 308. Modulationof this impedance state (e.g., chains of impedance pulses of varyingdurations) can be used to send more complex messages between the twoswitches so that, for example, the second switch can detect a problem,and take over, or request that the communications between the firstswitch and the third device proceed at a different communications speed,or the like.

Turning now to FIGS. 20 and 21, FIG. 20 is a graph showing the voltageof the VTERM signal (corresponding to the impedance modulation signal)plotted against time and FIG. 21 is a graph of the difference of thevoltages received at the first switch plotted against time. As caneasily be seen, when VTERM is asserted, the standard Ethernet voltagepulses have less measured amplitude than when VTERM is not asserted.Accordingly, this signaling technique may be used to send a flag for arequest for a direct conversation among both switches to negotiate linkstatus, master-slave relationship, or anything else using packets, FLP(Ethernet Fast Link Pulses), lower frequency pulses, proprietarysymbols, and the like.

FIG. 22 is a process flow diagram illustrating a first portion 330 of aprocess for resolving communication faults in accordance with anembodiment of the present invention. FIG. 23 is a process flow diagramillustrating a second portion 332 of a process for resolvingcommunication faults in accordance with an embodiment of the presentinvention. At 334 the first switch is communicating normally with thethird device, such as an Internet telephone, or the like. Suchcommunications generally include the periodic transmission of varioustypes of well-known packets and/or pulses. The absence of such packetsand/or pulses can indicate trouble such as a locked-up component, failedcomponent, or the like.

At 336 the second switch monitors communications between the firstswitch and the third device as described above.

At 338 if a fault is detected, control passes to 340 and if not, controlpasses back to 336. A fault detection may be due to the lack of datapacket traffic, fast link pulses, other link pulses, link beat signals,discovery protocol packets, lack of inline power, or any other conditionof interest on the wire.

At 340 the second switch initiates a back off request. It does this byloading the line by setting its impedance termination to a lowerimpedance than its normal high-impedance state. A switch aware of thiscapability for impedance communication that is still alive (as opposedto failed and unable to respond) may respond by backing off and goinginto FLP mode to carry out an auto negotiation process among the threedevices to establish status, control, master-slave relationship, and thelike. In many situations, this step will clear the fault andcommunications may proceed. If not, control passes to 342.

At 342 the second switch and the first switch communicate to attempt toresolve the fault. The resolution process for fault resolution isdenoted 344 and is described in more detail in FIG. 23.

At 346, one approach to fault resolution is to try lower speeds. In mostcases, while higher speeds are desirable, lower speeds are preferable toa complete link failure and many activities can be carried outsuccessfully at lower speeds. So, assuming that the link was initiallyset at a speed of 1000 MB/sec (1000 Base T) it may be re-tried at 100Base T (348, 350) and 10 Base T (354, 356) before deciding to try to letthe second switch take over the link (358, 360). In each case, some kindof notification should be sent to a network control point to assist inultimate resolution of the detected problem (352, 362). When the secondswitch takes over the link to establish a replacement connection, thereplacement connection may be at a speed which is the same or differentfrom the original link speed between the first switch and the thirddevice. In most situations, the replacement connection will be at alower speed than the original connection speed, typically because thesecond switch to third device connection will only be able to support alower speed communication rate, such as 10 Base T as opposed to a 100Base T or 1000 Base T communication rate existing between the firstswitch and the third device.

Where the detected problem is a loss of inline power, several optionsare available. First, as illustrated in FIG. 24, one option is toprovide inline power over both the first switch and the second switch.If power management is of no concern, both can be active at all times.Normally, however, power management is of concern, thus the first switch(main power supply) can be set to provide inline power at one voltagelevel and the second switch (backup power supply) can be set to provideinline power at a slightly different voltage level (e.g., by droppingthe voltage through one or more diodes) so that current flows from thefirst switch while it is operating, and when it stops supplying inlinepower because its voltage has dropped significantly (or to zero) thenthe other switch will automatically and relatively instantly supplyinline power. In the FIG. 24 embodiment, one diode is provided at theMain Power Supply (PSE 14 a′) to avoid power flowing into it and twodiodes are provided at the Backup Power Supply (PSE 14 a″) to set it ata lower voltage. Those of ordinary skill in the art will now recognizethat many other voltage setting mechanisms such as voltage regulators,different numbers of diodes or transistors, and the like, may be used toaccomplish the same or an equivalent effect.

Alternatively, when providing backup inline power, it may be desirableto use impedance communications to ask the first switch to formally shutdown its inline power circuitry so that it does not come back up whilethe backup inline power is active. This would be desirable, perhaps,where one does not want to make too much current available.

While embodiments and applications of this invention have been shown anddescribed, it will now be apparent to those skilled in the art havingthe benefit of this disclosure that many more modifications thanmentioned above are possible without departing from the inventiveconcepts disclosed herein. Therefore, the appended claims are intendedto encompass within their scope all such modifications as are within thetrue spirit and scope of this invention.

What is claimed is:
 1. A method for providing inline power between afirst and a second network device associated with a first node of anetwork and a third network device associated with a second node of thenetwork, in a wired data telecommunications network, the methodcomprising: providing first inline power at a first voltage level fromthe first network device to the third network device; providing secondinline power at a second non zero voltage level different from the firstvoltage level from the second network device to the third networkdevice; and selecting one of the first inline power and the secondinline power at the third network device in response to the voltagelevel associated therewith; wherein the first voltage level is a firstnon-zero DC voltage; wherein the second voltage level is a secondnon-zero DC voltage; and wherein the first non-zero DC voltage isdifferent from the second non-zero DC voltage.
 2. A method for providinginline power between a first and a second network device associated witha first node of a network and a third network device associated with asecond node of the network, in a wired data telecommunications network,the method comprising: providing inline power at a first voltage levelfrom the first network device to the third network device; coupling theinline power from the first network device to the second network devicethrough a Y-device; monitoring the first network device at the secondnetwork device; and detecting at the second network device a conditionof the first network device; initiating, responsive to said detecting,an impedance modulated signal from the second network device onto thewired data telecommunications network; receiving at the first networkdevice, the impedance modulated signal; and backing off inline powerprovided between the first network device and the third network devicein response to said receiving.
 3. The method of claim 2, furthercomprising: supplying inline power at a second voltage level from thesecond network device to the third network device.
 4. The method ofclaim 3, wherein: said second voltage level is lower than said firstvoltage level.
 5. A method for providing inline power between a firstand a second network device associated with a first node of a networkand a third network device associated with a second node of the network,in a wired data telecommunications network, the method comprising:providing inline power at a first voltage level from the first networkdevice to the third network device; coupling the inline power from thefirst network device to the second network device through a Y-device;monitoring the first network device at the second network device; anddetecting at the second network device a condition of the first networkdevice; initiating, responsive to said detecting, an impedance modulatedsignal from the second network device onto the wired datatelecommunications network; receiving at the first network device, theimpedance modulated signal; and backing off inline power providedbetween the first network device and the third network device inresponse to said receiving.
 6. The method of claim 5, furthercomprising: supplying inline power at a second voltage level from thesecond network device to the third network device.
 7. The method ofclaim 6, wherein: said second voltage level is lower than said firstvoltage level.
 8. The method of claim 6, wherein the second voltagelevel is non zero and different from the first voltage level.
 9. Themethod of claim 5, wherein the providing inline power between a firstand a second network device includes a common line portion for providingthe second voltage level and the first voltage level to connect thefirst node to the second node.
 10. A method for providing inline powerbetween a first and a second network device associated with a first nodeof a network and a third network device associated with a second node ofthe network, in a wired data telecommunications network, the methodcomprising: providing first inline power at a first voltage level fromthe first network device to the third network device; providing secondinline power at a second voltage level from the second network device tothe third network device; and selecting one of the first inline powerand the second inline power at the third network device in response tothe voltage level associated therewith; wherein the first voltage levelis a first non-zero DC voltage; wherein the second voltage level is asecond non-zero DC voltage; and wherein the first non-zero DC voltage isdifferent from the second non-zero DC voltage.
 11. The method of claim10, wherein a network cable extends along a network pathway between thefirst node and the second node to connect to a single network connectorof the third network device of the second node; wherein providing firstinline power at the first voltage level from the first network device tothe third network device includes conveying the first DC voltage throughone set of conductors of the network cable to the single networkconnector of the third network device of the second node; and whereinproviding second inline power at the second voltage level from thesecond network device to the third network device includes conveying thesecond DC voltage through another set of conductors of the network cableto the single network connector of the third network device of thesecond node.
 12. The method of claim 11, further comprising: afterselecting one of the first inline power and the second inline power atthe third network device, exchanging data communications signals betweenthe third network device and at least one of the first network deviceand the second network device through the network cable while the thirdnetwork device receives inline power through the network cable.
 13. Themethod of claim 12, wherein exchanging data communications signalsbetween the third network device and at least one of the first networkdevice and the second network device includes: exchanging datacommunications signals between the third network device and the firstnetwork device during a first time period, and exchanging datacommunications signals between the third network device and the secondnetwork device during a second time period after the first time period.14. The method of claim 11, further comprising: outputting a first copyof a data communications signal from the first network device, andconcurrently outputting a second copy of the data communications signalfrom the second network device; and wherein only one of the first copyof the data communications signal and the second copy of the datacommunications signal passes through the network cable to the singlenetwork connector of the third network device of the second node. 15.The method of claim 10, wherein a network pathway extends between thefirst node and the second node, the network pathway including (i) anintermediate device, (ii) a first network cable interconnected between asingle network connector of the first network device of the first nodeand the intermediate device, (iii) a second network cable interconnectedbetween a single network connector of the second network device of thefirst node and the intermediate device, and (iv) a third network cableinterconnected between the intermediate device and a single networkconnector of the third network device of the second node; whereinproviding first inline power at the first voltage level from the firstnetwork device to the third network device includes conveying the firstDC voltage through the first network cable, the intermediate device andthe third network cable to the single network connector of the thirdnetwork device of the second node; and wherein providing second inlinepower at the second voltage level from the second network device to thethird network device includes conveying the second DC voltage throughthe second network cable, the intermediate device and the third networkcable to the single network connector of the third network device of thesecond node.
 16. The method of claim 11, further comprising: outputtinga first copy of a data communications signal from the single networkconnector of the first network device of the first node to theintermediate device, and currently outputting a second copy of the datacommunications signal from the single network connector of the secondnetwork device of the first node to the intermediate device; and whereinthe intermediate device is constructed and arranged to convey only oneof the first copy of the data communications signal and the second copyof the data communications signal through the third network cable to thesingle network connector of the third network device of the second node.